Hydraulic machines such as excavators, dozers, loaders, backhoes, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to an engine-driven pump of the machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators and a connected work tool.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, there may be times when the hydraulic machine is idle and yet still operational. For example, during a truck loading cycle, when an excavator finishes loading a first truck, the excavator must wait for the first truck to depart and a second truck to arrive before additional loading tasks can be completed. And during this time, the engine of the machine may still be turned on (often at high speeds) and needlessly consuming fuel. In these situations, it may be beneficial to selectively turn the engine off to consume fuel.
However, restarting the engine can be harsh on the machine's electrical circuit and cause delays in the work cycle of the machine. Specifically, the electrical circuit could be called on to restart the engine hundreds of times during a particular work shift. In some applications, this overuse of the electrical circuit could cause premature wear and/or failure. In addition, it may take some time for the engine to be turned on and ramp up to required speeds. And this time delay could result in loss productivity and/or become a nuisance for the operator.
An exemplary system for starting and stopping a wheel loader is disclosed in U.S. Pat. No. 8,209,975 that issued to Persson et al. on Jul. 3, 2012 (“the '975 patent”). Specifically, the '975 patent discloses a hydraulic system operable in an energy saving mode during periods of low activity. Examples of such periods include when there is no demand for hydraulic pressure from an implement of the wheel loader, or when an operator of the wheel loader is not providing any control input. When either of these conditions is detected, the system checks to see if enough high pressure fluid has been collected within an accumulator to enable restarting of an engine. When enough high pressure fluid has been collected, the engine is shut down to conserve fuel. If any activity requiring hydraulic pressure (such as input from the operator) is thereafter detected, the energy saving mode is interrupted. This will cause the collected fluid to be directed from the accumulator through a motor that restarts the engine. At the same time, the motor also drives a pump that directs pressurized fluid to the implement so that the operator need not wait for the engine to restart, as fluid pressure from the accumulator is immediately available for controlling and operating the implement. When normal engine operation is resumed, the pump is driven by the engine to begin pressurizing the accumulator with high-pressure fluid.
Although the system of the '975 patent may improve engine restarting through the use of accumulated high-pressure fluid, the system may still be less than optimal. In particular, there may be instances when the engine is shutdown and the accumulator does not contain sufficient fluid to restart the engine. In these instances, the system of the '975 patent may be rendered useless. Further, directing accumulated fluid through the motor to drive the pump and supply fluid to the implement during engine restart may be inefficient. Further, by refilling the accumulator only with fluid from the pump may be wasteful.
The disclosed machine is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.